Voyager over the “heliocliff,” but Solar System transition mysterious

It's left the solar wind behind, but we're not sure what region it's in.

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun. You could also place it at the orbit of the last dwarf planet that roams the Kuiper belt. But if you want to define it where the Sun's energy directly affects the environment, then you'd place it at the edge of the heliosheath, where the solar wind and the Sun's magnetic field fall off, and the environment is dominated by the energetic particles of the interstellar medium.

That boundary, called the heliopause, was approached by the most distant man-made object around: Voyager 1. In December, researchers held a press conference to announce that, rather than crossing a clean boundary, the probe had entered a region near this edge that nobody had predicted. Today, the paper describing these results was released, and thanks to some public relations confusion, many outlets reported that the probe had left the Solar System entirely.

The evidence of a boundary is very clear in the chart shown below, which maps the particles that Voyager has recorded in its environment over the last year. In blue are low-energy protons emitted by the Sun itself. These fluctuated for a few weeks before dropping off a cliff by over 99 percent. The authors of the paper have termed this transition the "heliocliff."

As Voyager 1 approached the cliff, high energy electrons (red) and protons (black) were rising; these come from the interstellar medium and are largely blocked by the force of the solar wind. Once the cliff is passed, however, these energetic particles reach new highs and stay here. By this measure, Voyager 1 has left a key area of solar influence and is now sampling the interstellar medium for the first time. The authors of the paper are so excited by this that they use the terms "extraordinary," "remarkable," and "holy grail" to describe the transition—that sort of terminology doesn't appear often in the scientific literature.

Enlarge/ After several partial crossings, Voyager 1 is clearly experiencing the interstellar medium.

But has Voyager actually passed beyond the heliopause? The authors argue that it has: "If [these] intensities continue to remain at their present levels, then indeed this 'heliocliff' region displays many of the properties of a 'classical' heliopause, perhaps a much more impressive barrier to inward and outward transport of energetic particles than would have been anticipated." However, what's missing is a similar transition in the magnetic field lines from the Sun. Those were expected to change at the heliopause as well, and until they do, all we can say is that Voyager is passing through an unpredicted region at one of the edges of our Solar System.

Unfortunately, a press officer seems to have gotten a bit ahead of the data, as the initial announcement of the paper's release was entitled "Voyager 1 has left the solar system, sudden changes in cosmic rays indicate." That led to a predictable number of news stories that echoed this language. Since then, however, the American Geophysical Union has updated its release, which now says only that "Voyager 1 has entered a new region of space." Which is something we knew as far back as the press conference in December.

The paper at least puts the data in front of other scientists. And its acknowledgements contain a touching tribute to one of the original Voyager team members, which we'll quote in its entirety:

This article was conceived by our Voyager colleague, Frank McDonald, who is no longer with us. Frank, we have been working together for over 55 years to reach the goal of actually observing the interstellar spectra of cosmic rays, possibly now achieved almost on the day of your passing. You wanted so badly to be able to finish this article that you had already started. Together we did it. Bon Voyage!

"Unfortunately, a press officer seems to have gotten a bit head of the data, as the initial announcement of the paper's release was entitled, "Voyager 1 has left the solar system, sudden changes in cosmic rays indicate." That led to a predictable number of news stories that echoed this language."

Knowing how badly the local news media butchers any type of science news, this is going to end up being reported that "Voyager has left the Universe"

That is really a wonderful looking graph. But, my reading of the conclusion of the article is that it would be an even better graph with a fourth line that represents "magnetic field lines from the Sun". Where this fourth line would not have any dramatic movement in it.

That is really a wonderful looking graph. But, my reading of the conclusion of the article is that it would be an even better graph with a fourth line that represents "magnetic field lines from the Sun". Where this fourth line would not have any dramatic movement in it.

Damn, beat me to it. These are charged particles, therefore influenced by magnetic lines of flux. Normally the density of these taper with distance but might there be a mechanism that might account for the sudden increase in density of said flux?

I'm not an astrophysicist by any stretch of the imagination, but could field compression account for this?

This new makes the everyday struggles and concerns look petty and insignificant.

And it makes me wanting to build probes, lots of probes to probe the whole solar system out and even send stuff out of the Solar system so we can replace those Voyagers and Pioneer once they will run out of energy.

Old McDonald had a cliff, e-i-e-i-oAnd on that cliff there was a Voyager, e-i-e-i-o.With a beep beep here and a beep beep there, here a beep, there a beep, everywhere a beep beep.Old McDonald had a cliff, e-i-e-i-o

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun...

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I don't really think this is brilliantly phrased, mostly because it implies that the gravitational field of the Sun magically goes to zero at the end of the Oort cloud. However, Gravity's potential goes like 1/r, so the only place where you can't feel the gravitational attraction of the Sun is as r goes to infinity -- i.e. when you're infinitely far away!

Does anyone know if Frank McDonald was told of the heliocliff, before he died? He died August 31st.

Yes. He was one of the two authors credited on DOI: 10.1002/grl.50383 (the paper this article cites), and the fact his co-author said "finish" the article indicates he started writing about the heliocliff before he passed.

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun...

.

I don't really think this is brilliantly phrased, mostly because it implies that the gravitational field of the Sun magically goes to zero at the end of the Oort cloud. However, Gravity's potential goes like 1/r, so the only place where you can't feel the gravitational attraction of the Sun is as r goes to infinity -- i.e. when you're infinitely far away!

In the interest of taking things too far I disagree. The sun absolutely has a gravitational range. I would peg it at roughly 4.6 billion light years or just a little beyond the Bullet Cluster(which is 3.7 billion light years away who knows where it is now though). Honestly though after a certain point the gravitational pull of the sun becomes weaker than the Milky way or neighboring stars. At this point we consider an object outside of the Suns gravitational influence because other bodies have more influence.

However, Gravity's potential goes like 1/r, so the only place where you can't feel the gravitational attraction of the Sun is as r goes to infinity -- i.e. when you're infinitely far away!

Influence of gravity is also limited by the speed of light, and since our sun isn't infinitely old there's lots of places in the universe where its (utterly miniscule and completely dismissable) influence has not yet manifested itself. And yeah, our sun's gravitational influence is of course lost so far below the 'background noise floor' even quite close to us within our own milky way galaxy that any such talk is just nonsense really.

Given the technical simplicity (by today's standards) of the Voyager probes, I really think we should be sending deep-space experiments out about once per decade. Naturally with state-of-the-art instruments, but more importantly built for longevity and with trajectories designed to speed them out of the solar system as fast as possible, possibly taking a few snapshots on the way.

Missions like this would not be expensive or complicated, but would provide a lot of data about interstellar space from direct observation.

Voyager 1's journey is truly one of the most fascinating science stories right now. It's one of the most amazing accomplishments in human history, and it testing a boundary that is truly hard to conceive. How far can it go? That's what I want to know...will it ever reach the "edge"...what is the edge exactly?

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun...

.

I don't really think this is brilliantly phrased, mostly because it implies that the gravitational field of the Sun magically goes to zero at the end of the Oort cloud. However, Gravity's potential goes like 1/r, so the only place where you can't feel the gravitational attraction of the Sun is as r goes to infinity -- i.e. when you're infinitely far away!

The Oort cloud is held in place by the Sun's gravitational field.Any thing outside of the Oort cloud is not affected in any noticeable way, if it were, it would be part of the Oort cloud.Nobody claims that the Sun's gravitational pull goes to zero, only that, at a certain distance, the pull is no greater than the pull off all other gravitational fields in our galaxy.

Where does our Solar System end? If you define it in terms of the Sun's gravitational influence, then it's the edge of the Oort cloud, a collection icy bodies that stretches over two light years from the Sun...

.

I don't really think this is brilliantly phrased, mostly because it implies that the gravitational field of the Sun magically goes to zero at the end of the Oort cloud. However, Gravity's potential goes like 1/r, so the only place where you can't feel the gravitational attraction of the Sun is as r goes to infinity -- i.e. when you're infinitely far away!

Not to mention that the Oort cloud is still a fictitious concept. As far as gravity is concerned you could argue that gravity from one object disappears only when gravity from another object over powers it (or cancels out it's "pull").

It may be possible that some phenomenon of space-time would eventually cancel out gravity once it reached a specific minimum, but we still don't know exactly how those interact to make that calculation (as far as I know).

The Oort cloud is held in place by the Sun's gravitational field.Any thing outside of the Oort cloud is not affected in any noticeable way, if it were, it would be part of the Oort cloud.Nobody claims that the Sun's gravitational pull goes to zero, only that, at a certain distance, the pull is no greater than the pull off all other gravitational fields in our galaxy.

The only reason the Oort cloud would be held in place and not falling toward the Sun is because it is revolving around the Sun. Theoretically, objects even twice as far from the Sun could still fall toward it if they were relatively motionless to the Sun. The exception would be if said object was in a stronger gravitational field.

That makes me wonder, when (at Oort cloud distances) does the gravitational field of a standard comet become greater than the Sun's gravity field?